1. Field of the Invention
The present invention relates to improvements in an electroacoustic transducer which incorporates a piezoelectric element disposed on a diaphragm for converting sound waves or acoustical signals to electric waves or signals, and an amplifier for amplifying the electric signals.
2. Discussion of the Prior Art
As an electroacoustic device such as a microphone, there is known an electroacoustic transducer which uses a piezoelectric element disposed on a diaphragm to convert acoustical signals to electric signals, and an amplifier for amplifying the obtained electric signals. An example of this type of electroacoustic transducer is shown in FIG. 3, wherein one electrode 14 on one of opposite surfaces of a piezoelectric element 12 disposed on a diaphragm 10 is connected through an input line 16 to an amplifier 18, while the other electrode 20 on the other surface of the element 12 is connected to an earth 22. Oscillation of the diaphragm 10 by sound waves causes the piezoelectric element 12 to produce an electric potential which varies with the amplidude of the oscillation of the diaphragm. As a result, an electric signal representative of a difference between the produced electric potential and a reference potential of the earth 22 is applied to the amplifier 18, which amplifies the electric signal by a predetermined amplification factor. The amplified signal is delivered from an output terminal 24 of the device.
The use of piezoelectric ceramics as a piezoelectric element has been proposed, for increased sensitivity and freedom of design and for reduced weight of the element. Examples of the piezoelectric ceramics include BaTiO.sub.3, PZT (PbZrO.sub.3 -PbTiO.sub.3), and a three-element mixture consisting of PZT and a compound perovskite composition. For instance, the use of these piezoelectric ceramics makes it possible to detect very weak sound waves such as heartbeat sounds, or minimize the thickness and weight of a detecting portion disposed on the diaphragm of an electroacoustic transducer. Thus, the piezoelectric ceramic materials have various advantages.
However, since a piezoelectric element made of such piezoelectric materials has an extremely high output impedance, it is necessary to use on amplifier which has an accordingly high input impedance. Therefore, the electroacoustic transducer using such high-impedance piezoelectric element and amplifier tends to easily pick up noises induced in the input line. A known method to avoid this tendency is to use shielded wires as the input line. This method is not completely satisfactory in preventing the pickup of the induced noises.
In the meantime, it is known to use a differential amplifier as means for removing the induced noises from the electric signals. An example of an arrangement using such a differential amplifier is shown in FIG. 4, wherein a pair of electrodes 32, 34 on opposite surfaces of a piezoelectric element 30 are connected through respective input lines 36, 38 to a pair of input terminals of a differential amplifier 40. In this arrangement, noises of equal levels induced in the two input lines 36, 38 may be offset or cancelled by each other by the differential amplifier 40.
In this case, however, there is no fixed reference for the electric potential generated by the piezoelectric element 30, and consequently the level of the electric signal applied to the differential amplifier 40 through the input lines 36, 38 may be cause to flucture by the induced noises or other factors. Thus, there exists a possibility that the level of the input signals applied to the differential amplifier 40 may not be held within the predetermined range of the amplifier, causing distortion of the output signal. To overcome this inconvenience, one of the two input lines 36, 38 must be connected to a reference potential such as an earth. This arrangement is substantially the same as the circuit shown in FIG. 3. Therefore, it is not possible to obtain the intended effect of the arrangement of FIG. 4 of offsetting the induced noises by differential amplification.